Reference-free thyroid uptake measurement

Gamma camera-specific reference standards for radioactive iodine uptake measurements

BACKGROUND

Current guidelines of the radioiodine uptake (RAIU) test allow the use of different equipment, isotopes, activity and region-of-interest (ROI). We evaluated presence and extent of these differences in clinical practice and evaluated the effect of some of these variations on RAIU outcomes. Also, gamma camera-specific reference standards were calculated and retrospectively compared with measurements obtained during clinical RAIU tests.

MATERIALS AND METHODS

First, questionnaires were sent to Dutch nuclear medicine departments requesting information about equipment usage, isotope, isotope formulation, activity and measurement techniques. Secondly, a neck phantom containing a range of activities in capsule or water-dissolved formulation was scanned. Counts were measured using automatic ROI, square box ROI or all counts in the image. Thirdly, clinical RAIU data were collected during 2015-2018 using three different gamma cameras. Reference standards for each scanner were calculated using regression analysis between reference activity and measured counts. Uptake measurements using this gamma camera-specific reference standard were compared with original measurements.

RESULTS

The survey demonstrated significant differences in isotope, isotope formulation, activity, use of neck phantoms, frequency and duration of reference measurements, distance to collimator, use of background measurements and ROI delineation. The phantom study demonstrated higher counts for the water-dissolved formulation than capsules using both automatic and square box ROI. Also, higher counts were found using a square box ROI than an automatic ROI. The retrospective study showed feasibility of RAIU calculations using camera-specific reference standards and good correlation with the original RAIU measurements.

CONCLUSIONS

This study demonstrated considerable technical variation in RAIU measurement in clinical practice. The phantom study demonstrated that these differences could result in differences in count measurements, potentially resulting in different dose calculations for radioactive iodine therapy. Retrospective data suggest that camera-specific reference standards may be used instead of individual reference measurements using separate activity sources, which may thus eliminate some sources of variation.

Reduction of thyroid radioactive iodine exposure by oral administration of cyclic oligosaccharides

Alpha-cyclodextrin, a six D-glucose cyclic oligosaccharide, has several applications in food and pharmaceuticals, but has also been reported to retain iodine in a stable manner for 16 months. Radioactive iodine, which may cause thyroid cancer and hypofunction, must be properly managed. If the absorption of radioactive iodine is suppressed, it can be expected to lead to a reduction in thyroid exposure. This study clarified the inhibition of radioactive iodine absorption by the oral administration of α-cyclodextrin in a murine model using direct measurement of single photon emission computed tomography. The uptake of radioactive iodine into the thyroid gland in mice administered with radioactive iodine and an α-cyclodextrin solution was approximately 40% lower after 24 h. The finding that oral uptake of α-cyclodextrin has an inhibitory effect on the transfer of radioactive iodine to the thyroid gland has potential for application in many fields such as food, pharmaceuticals, nuclear emergency preparedness, and medicine.

Calculation of therapeutic activity of radioiodine in Graves' disease by means of Marinelli's formula, using technetium (99mTc) scintigraphy

The therapeutic activity of ¹³¹I administered to patients with Graves' disease can be calculated by means of Marinelli's formula. The thyroidal iodine uptake (¹³¹IU_max) needed for the calculation is usually determined with the use of ¹³¹I. The purpose of the paper was to estimate ¹³¹IU_max on the basis of technetium uptake in the thyroid at 20 min (^99mTcU_20min). Eighty patients suffering from Graves' disease were qualified for radioiodine therapy with measurement of fT₄, fT₃, thyroid-stimulating hormone and its receptor (TRAb). Prior to the treatment, all the patients were euthyroid. ¹³¹IU_max for each patient was determined according to the levels of ¹³¹I after 24 h (¹³¹IU_24h), while effective half-life (T _eff) according to the measurements of ¹³¹IU_24h and ¹³¹I uptake after 48 h (¹³¹IU_48h). Additionally, on the day before measuring ¹³¹IU_24h, ^99mTcU_20min was calculated for each patient. It was demonstrated that there existed a correlation, with statistical significance at p < 0.05, between the following pairs of values: TRAb and ¹³¹IU_24h, TRAb and ^99mTcU_20min, and ^99mTcU_20min and ¹³¹IU_24h. The interdependence between ¹³¹IU_24h and ^99mTcU_20min at the level of significance p < 0.05 is described by the following algorithms: ¹³¹IU_24h = 17.72 × ln (^99mTcU_20min) + 30.485, if TRAb < 10 IU/ml, and ¹³¹IU_24h = 18.03 × ln (^99mTcU_20min) + 38.726, if TRAb > 10 IU/ml. It is possible to predict thyroid iodine uptake ¹³¹IU_24h in Graves' disease on the basis of measuring the uptake of ^99mTcU_20min. This shortens the time necessary for diagnosis and enables the calculation of ¹³¹I activity using Marinelli's formula.